1. Field of the Invention
The present invention relates to a monolithically integrated charge transfer circuit having a signal scanner, a first clock pulse controlled metal-insulator-semiconductor (MIS) charge transfer system which is supplied with charge packets via the signal scanner and a clock pulse controlled output stage which is connected to the MIS charge transfer system and which serves for the restoration of a signal which is constant in time.
2. Description of the Prior Art
Such monolithically integrated charge transfer circuits are, for example, described in "Der Elektroniker", No. 3/1978, pp. EL7-EL15, whereas the essentials concerning MIS charge transfer systems having clock pulse control may be found in "Der Elektroniker", No. 2/1978, pp. EL3-EL6. The embodiment of such an output stage for MIS charge transfer systems is to be represented in the following for charge coupled circuits (CCD circuits), and nevertheless it is applicable in the same manner to bucket brigade device circuits (BBD circuits).
In the case of the transmission of signals by means scanning systems, the scanning brings about a periodicity of the frequency spectrum of the scanned signal, which must be done away with in the case of a conversion into signals which are constant in time by means of suitable filters, since the higher spectral components interfere in the case of multiples of the scanning frequency for many cases of application. This means that one must solve the problem of regaining a band-limited signal from the output signal of a charge transfer circuit, which output signal is formed by the scanning values, whereby the higher spectral components which arose by means of scanning are adequately attenuated. For the interpolation of the scanning values, as a rule, a scan holding element and a subsequently connected analog low pass filter are provided. In many cases, a RC filter is insufficient for the low pass filter which is to be used, so that one must use LC filters. However, with such structure, one loses the possibility of integrating the filter monolithically upon a common semiconductor chip with the remaining parts of a charge transfer circuit. A completely integratable solution of the problem is nevertheless of great significance for CCD circuits which, for example, form filters or time delays.
There are two possibilities for reducing the selection requirements placed upon a low pass filter: (1) increasing the scanning frequency f.sub.A of the CCD; or (2) post-connecting a further CCD low pass filter which functions with the scanning frequency f'.sub.A to the first CCD, whereby the scanning frequency f'.sub.A of the low pass filter is larger than the scanning frequency f.sub.A of the first CCD. Then the following analog low pass filter can be designed very simply, or perhaps can be completely omitted.
However, a higher scanning frequency of a CCD installation [case (1)] in the case of the same system requirements, increases proportionally the number N of the charge transport elements and, therefore, increases the surface requirement for the semiconductor chip and, as a rule, leads to poorer system characteristics by means of raised charge transport losses (.epsilon.N). Also, the expense of a following CCD low pass filter of higher scanning frequency f'.sub.A is relatively high, conditioned by the clock pulse generation.